In the past, direct volts calibrators were very complex instruments which required highly trained operators. To obtain highest accuracy for use on a daily basis required extremely difficult and time consuming calibration procedures. Often, the instrument had to be partially disassembled in order to reach the zero setting potentiometers and a great deal of skill was required in zeroing the instrument.
Further, it was very difficult to obtain setting accuracy to within half a microvolt and it was very time consuming to keep track of the instrument's performance so that problems could be located before a catastrophic failure occurred.